plot: Y-range value display bug

running the following program:

package main

import (
	"flag"
	"log"

	"github.com/gonum/plot"
	"github.com/gonum/plot/plotter"
	"github.com/gonum/plot/vg"
)

func main() {
	v := flag.Float64("v", 1300, "y-value")
	flag.Parse()

	p, err := plot.New()
	if err != nil {
		log.Fatal(err)
	}

	p.Y.Label.Text = "RPMs"

	var data = make(plotter.XYs, 10)
	for i := range data {
		data[i].X = float64(i)
		data[i].Y = *v
	}

	lines, points, err := plotter.NewLinePoints(data)
	if err != nil {
		log.Fatal(err)
	}
	p.Add(points, lines)
	p.Add(plotter.NewGrid())

	err = p.Save(10*vg.Centimeter, 10*vg.Centimeter, "foo.png")
	if err != nil {
		log.Fatal(err)
	}
}

gives the following plot:

ie: while the data line is at the correct location, the Y-axis labels are wrong (1300 is repeated twice, 1301 and 1299 aren't probably consistent).

this doesn't occur when I run with, say, 1401 or 130 as a value, but this bug also occurs for 1201, 1400 and 1501.

I suspect an issue in the rounding+formatting of the axis labels.